Abstract
The intervertebral disc (IVD) plays a main role in absorbing and transmitting loads within the spinal column. Degeneration alters the structural integrity of the IVDs and causes pain, especially in the lumbar region. The objective of this study was to investigate non-invasively the effect of degeneration on human 3D lumbar IVD strains (n = 8) and the mechanism of spinal failure (n = 10) under pure axial compression using digital volume correlation (DVC) and 9.4 Tesla magnetic resonance imaging (MRI). Degenerate IVDs had higher (p < 0.05) axial strains (58% higher), maximum 3D compressive strains (43% higher), and maximum 3D shear strains (41% higher), in comparison to the non-degenerate IVDs, particularly in the lateral and posterior annulus. In both degenerate and non-degenerate IVDs, peak tensile and shear strains were observed close to the endplates. Inward bulging of the inner annulus was observed in all degenerate IVDs causing an increase in the AF compressive, tensile, and shear strains at the site of inward bulge, which may predispose it to circumferential tears (delamination). The endplate is the spine's “weak link” in pure axial compression, and the mechanism of human vertebral fracture is associated with disc degeneration. In non-degenerate IVDs the locations of failure were close to the endplate centroid, whereas in degenerate IVDs they were in peripheral regions. These findings advance the state of knowledge on mechanical changes during degeneration of the IVD, which help reduce the risk of injury, optimize treatments, and improve spinal implant designs. Additionally, these new data can be used to validate computational models.
Highlights
Three in four people will experience low back pain (LBP) at some point in their life and in terms of disability it is ranked first among the 291 conditions that were reviewed in a 2010 Global Burden of Disease study (Hoy et al, 2014)
Analyzing the photographs showed that non-degenerate intervertebral disc (IVD) were white in color, the nucleus pulposus (NP) appeared to be gelatinous, and usually there was no sign of annulus fibrosus (AF) tears or delamination
Our findings show that degeneration caused greater changes in axial, 3D minimum principal, and 3D maximum shear strains in the posterior AF than the anterior AF where no significant differences were seen in any strain components between degenerate and non-degenerate samples (Figure 5)
Summary
Three in four people will experience low back pain (LBP) at some point in their life and in terms of disability it is ranked first among the 291 conditions that were reviewed in a 2010 Global Burden of Disease study (Hoy et al, 2014). Degeneration causes severe changes in the mechanical response of IVDs (Adams et al, 1996), in terms of alterations in the distribution of forces through the vertebral bodies (VB) and endplates, affecting the mechanism of spinal fracture (Adams et al, 2006). Previous studies have shown that the mechanism of vertebral fracture is associated with the quality of the underlying trabecular bone, endplate thickness (Zhao et al, 2009), and endplate deflection (Jackman et al, 2014), the influence of 3D IVD strain patterns on failure mechanisms has not previously been investigated. Understanding the relationship between mechanical behavior of the IVD and degeneration is important for improving treatments for LBP patients, such as implants and tissue engineered IVDs
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